Connection between composites with non-compatible properties and method for preparation

ABSTRACT

A connection between composites with non-compatible properties and a method of preparing of such connections are provided. The composites comprise first and second type fibers, respectively, as well as resin. The connection comprises a transition zone between the composites having a layered structure. The transition zone may optionally comprise a transition member and the transition member may optionally be integrated with one or more of the composites. Examples of non-compatible properties where the present connection will be appreciated are great differences in stiffness, e.g. Young&#39;s modulus, or in coefficient of thermal expansion.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This is a division of U.S. patent application Ser. No. 10/548,060, filedSep. 6, 2005, which is a national stage of PCT/EP03/04171, filed Apr.22, 2003, now issued as U.S. Pat. No. 7,521,105.

FIELD OF THE INVENTION

The invention relates to fibre reinforced composites. In particular, theinvention relates to permanent connections between composite memberscomprising different types of reinforcement fibres.

BACKGROUND OF THE INVENTION

When designing structures of composite materials, the optimum materialwith regard to strength, weight, E-modulus and cost etc. is often notthe same for all parts of the structure. For example, in a spar for awind turbine blade the preferred material for the base part may be aglass fibre reinforced composite due to low cost and limited mechanicalrequirements whereas the preferred material for a load bearing outerpart like a flange may be carbon fibre reinforced composite due to thehigher stiffness and lower weight. The physical properties, for examplestiffness and thermal expansion, are very different, however, and it isthe general conception in the art that such parts cannot be effectivelyconnected.

OBJECTS OF THE INVENTION

It is the object of the invention to provide an effective permanentconnection between two composite members for example being reinforcedwith different reinforcement fibres.

It is another object of the invention to provide a method for preparingsuch connections.

DISCLOSURE OF THE INVENTION

The above and more objects are realised by the invention as describedand explained in the figures, preferred embodiments and claims.

The connection according to the invention may provide a stable relativefixing of a first composite member and a second composite member havingsubstantially the same properties. It will be appreciated, however, thatthe connections may also be between composite members having ratherdifferent properties. For example composites being reinforced withcarbon fibres, which typically have coefficients of thermal expansion(hereinafter CTE) near of even below zero, may be connected tocomposites being reinforced with glass fibres, which have asubstantially higher CTE.

The invention concerns a connection between a first composite membercomprising first type fibres and a first resin, and a second compositemember comprising second type fibres and a second resin. The connectioncomprises a transition zone having a layered structure. By transitionzone is meant the volume of the connection that does not have the samecomposition and the same structure as the bulk of either of theconnected composite members. The transition zone comprises the volumebetween the two composite members and may in some cases also include apart of one or more of the composite members.

By first composite member is referred to the part of the final connectedstructure having a structure substantially corresponding to anequivalent separately prepared first composite member and vice versa fora second composite member. Any one of the composite members may hence beprepared partially or fully in advance of the formation of theconnection or the composite member may be prepared as an integrated partof the process of preparing the connection.

The volume between the composite members may optionally be fully orpartially occupied by a transition member. This transition member may beprepared prior to the establishing of the connection, it may be prepareddirectly in the transition zone as part of the establishing of theconnection or any combination of these.

Each of the composite members may, independently, be uncured,pre-consolidated, partially or fully cured at the time of establishingof the connection. Likewise, a transition member, which is partially orfully prepared prior to the establishing of the connection, may beuncured, pre-consolidated, partially or fully cured at the time of theestablishing of the connection. However, it is preferred that atransition is not fully cured at the time of the establishing of theconnection.

The connected structure may be established (i.e. laid up) in only oneoperation. By one operation is meant, that both the composite membersand the transition zone is prepared in connection to the other parts,typically but not necessarily followed by pre-consolidation or co-curingof the complete structure. This type of preparation is particularlyuseful when a transition member is not used.

Another way to establish the connection is to connect two members.Examples thereof may be:

Positioning composite members near or in contact with each other andestablishing a transition zone, optionally via a transition member,prepared ‘onsite’.

Connecting a composite member, which is prepared for connecting forexample by having an incorporated partial transition zone, to anothercomposite member which, optionally, is also prepared for the connection;

Connecting a composite member, which is prepared with a partially orfully integrated transition member, to another composite member which,optionally, is also prepared for connection;

Connecting a composite member to a transition member and preparing afurther composite member directly onto the transition member; etc.

Alternatively, the connection may be established by combining three ormore separate members (e.g. two composite members and one transitionmember, three composite members and no transition member, threecomposite members and one transition member, etc.).

Connections according to the invention may be used for connectingcomposite members in any direction relative to the main fibreorientation or orientations (e.g. parallel side by side, at an angle(orthogonal or any other) or parallel end to end). If fibres or layerscomprising fibres are interlaced into a composite member, it ispreferred, however, that the interlaced fibres are oriented at an angleto the interface between the composite members or at an angle to theinterface between the composite member and the transition member. Thiswill tend to increase the mechanical strength of the contact relative toa parallel orientation.

The term composite member herein denotes any type of composite materialcomprising fibres, cured or uncured, irrespective of the structure beinglayered or not. Pre-forms and pre-consolidated pre-forms—cured oruncured—are important subgroups of composite members. In a preferredembodiment, a first composite member comprises first type fibres andfirst type resin, and a second composite member comprises second typefibres and second type resin.

A transition zone as well as a transition member comprises a resin andfibres. The fibres may be provided in any suitable form including inprepregs, semi-pregs, woven or non-woven fabrics, mats, pre-forms,pre-consolidated pre-forms, individual or groups of fibres, tows,tow-pregs, etc. During lay-up (i.e. preparing up to the point beforeconsolidation and/or curing of the resin) of a transition zone or atransition member, resin need not be comprised in the layers comprisingfibres (e.g. a prepreg or semi-preg) or between the layers comprisingfibres. However, the resin should form a continuous matrix after thecuring. Resin need not be comprised in or between two adjacent layerscomprising fibres. In a preferred embodiment an adhesive may in thiscase be provided between at least some of such pairs of layers to atleast temporarily and at least partially fix the adjacent layerscomprising fibres.

By prepreg is meant a substantially or fully impregnated collection offibres, fibre tows, woven or non-woven fabric etc. By semi-preg is meanta partially impregnated collection of fibres or fibre tows. The partialimpregnation provides for enhanced removal of gas through or along thedry fibres during consolidation and/or curing. An example of a semi-pregis a layer of fibres (e.g. glass fibres or any other type of fibresmentioned herein) partially impregnated in the upper part and/or in thelower part. Woven and non-woven fabrics are collections of individualfibres or fibre tows which are substantially dry, i.e. not impregnatedby a resin. Fibre tows are bundles of a large number of individualfibres, e.g. 1,000's, 10,000's or 100,000's of fibres. Tow-pregs are atleast partially impregnated fibre tows.

It is within the scope of the invention to connect three or morecomposite members with the connection and the method according to theinvention as this is considered a collaboration of a number ofconnections according to the invention.

The transition zone or transition member may be prepared as a pre-formas described below. A pre-form is a composite material comprising fibresand—unless otherwise stated—an uncured resin. The fibres are preferablyprovided in layers of oriented fibres like for example individual orgroups of fibres, fibre tows, fibre tow-pregs, prepregs, semi-pregs,woven or non-woven fabrics or mats. Individual fibres, fibre tows andfibre tow-pregs may in some cases be advantageous over prepregs, sincethe individual fibres are less bounded and hence may rearrange easierduring subsequent processing. Furthermore, individual fibres, fibre towsand tow-pregs may be advantageous over prepregs in that they may beprovided in the pre-form with a greater freedom of mixing andorientation, the price is lower as well as the amount of waste may belower. The pre-form preferably comprises at least three layers oforiented fibres. Pre-forms having a higher number of layers like e.g. 4,5, 8, 10, 15, 20, 50, 100 or more layers may be used within the scope ofthe invention.

By fibres are hereinafter meant particles having an aspect ratio(length/equivalent diameter) of more than 10. By equivalent diameter ismeant the diameter of a circle having the same area as the crosssectional area of the particle. However, in a preferred embodiment, thefibres are continuous fibres, i.e. fibres that substantially run fromone edge of a pre-form or member to another. The properties of a fibrereinforced composite depend to a large extent on the properties of thefibres. However, the properties of different types of fibres varyconsiderably. For example, the coefficient of thermal expansion ofcarbon fibres is very low, and in some cases even negative. The firsttype fibres and the second type fibres may be any type of fibres havingan influence on the properties of the composite member; it is preferred,however, that the fibres are selected from the group consisting ofcarbon fibres, glass fibres, aramid fibres, synthetic fibres (e.g.acrylic, polyester, PAN, PET, PE, PP or PBO-fibres, etc.), bio fibres(e.g. hemp, jute, cellulose fibres, etc.), mineral fibres (e.g.Rockwool™, etc.), metal fibres (e.g. steel, aluminium, brass, copper,etc.), boron fibres and any combination of these.

In a preferred embodiment the first type fibres are carbon fibres andthe second type fibres are glass fibres or the first type fibres areglass fibres and the second type fibres are carbon fibres. It isparticularly interesting to connect composites with these fibre types ascarbon fibres are very stiff and light and glass fibres are veryaffordable. In another closely related embodiment, the first type fibresare carbon fibres and the second type fibres are glass fibres or viceversa and both the first and the second type resin are epoxy-based.

By carbon fibres is hereinafter meant fibres where the main component iscarbon. Hence, by this definition carbon fibres comprise fibres withgraphite, amorphous carbon or carbon nano-tubes. Thus, carbon fibresproduced via for example a polyacrylonitril-route or a pitch-based routeare comprised by this definition.

The fibres comprised in the transition zone and/or the transition memberand/or the composite members being connected may be a mixture of morethan one type of fibres. For example, a combination of glass fibres andcarbon fibres may be used, but any combination of two or more of thefibre types mentioned herein is feasible. The mixture may behomogeneous, with different concentrations in separate fibre layers orareas or with different concentrations of fibres within any fibre layer.Mixing of fibres may be advantageous, since this opens for tailoring ofmaterial properties, for example from a combinedstress/cost-perspective.

The resin for the composite members, the transition zone and theoptional transition member may be provided as liquid, semisolid or solidresin. The resin may be a thermoplastic or a thermosetting resin, it ispreferred to use a thermosetting resin, however, for reasons of chemicaland thermal stability as well as ease of processing. The resin may forexample be based on unsaturated polyester, polyurethane, polyvinylester,epoxy, thermoplastics or similar chemical compounds, includingcombinations of these. In a preferred embodiment the first type resinand the second type resin have substantially the same composition. Thisis preferred as it reduces compatibility problems.

In a preferred embodiment of the invention, the resin is provided as aliquid and the resin is introduced by Resin Infusion, Resin TransferMoulding, RTM, or Vacuum Assisted Resin Transfer Moulding, VARTM, intoan entity comprising several layers comprising fibres (e.g. fibre towsor any other suitable collection comprising fibres mentioned herein).The entity comprising fibres may for example be a transition zone, atransition member, a composite member or any combination of one or moreof these. Besides the layers comprising fibres, the entity may or maynot further comprise a resin and/or an adhesive. In one embodiment, theentity comprises two adjacent layers comprising fibres having noadhesive or resin between the layers prior to the introduction of theliquid resin as described previously in this section. In anotherembodiment, adhesive and/or resin is provided between all layerscomprising fibres in the entity prior to the introduction of the liquidresin as described previously in this section.

The main function of the adhesive is to immobilise the fibres as theyare placed on top of the adhesive. This can be achieved by having atacky adhesive, whereby the fibres stick to the tacky adhesive. Theadhesive may be any tacky material, or a solid with a tacky surface andthe adhesive may for example comprise polyester, polyurethane,polyvinylester, epoxy or similar compounds or a combination of these. Itis within the scope of the invention to use any material or combinationof materials having a tacky surface including solid materials with tackysurfaces. More than one type of adhesive may be used in one member ortransition zone. For example, it is within the scope of the invention touse the resin as an adhesive between layers of fibre tows, where a resinis provided, or to use a second type of resin below the first layer offibre tows.

In another preferred embodiment, the resin is a solid. An entitycomprising several layers of oriented fibre tows, which may optionallyhave been immobilised previously during fibre laying by an adhesive, anda solid resin system is heated under vacuum in order to prepare apre-consolidated or cured pre-form, which may be part of a transitionzone, a transition member or a composite member.

In a further preferred embodiment, the resin is a semisolid andfunctions both as resin and as adhesive, i.e. during fibre laying, theresin will immobilise the fibres and during subsequent processing, itfunctions as a matrix material.

The resin may comprise more than one system, for example the resin maycomprise two systems or even more systems. It may be advantageous to usemore than one resin system to be able to optimise the properties of theresin for the subsequent steps of processing, for example with respectto viscosity and timing/controlling of the curing process. These systemsmay or may not be based on the same type of resin, however, it ispreferred that such systems are based on the same type of resin such astwo or more epoxy-based systems. In another preferred embodiment, theresin types differ but the resins are compatible. In a further preferredembodiment, the resin comprises two substantially epoxy-based systems.The two epoxy-based systems may comprise a common component. The commoncomponent may for example be a common catalyst, a common amine componentor a common epoxy component, however, it is preferred that the commoncomponent is an epoxy component. A resin comprising two epoxy-basedsystems with a common epoxy component may comprise an amine-component ofa first epoxy-based system that will react to the common epoxy componentat a first relatively low temperature, such as below 50° C. andpreferably about room temperature. At this first temperature, a secondepoxy-based system is preferably non-reactive or the reaction takesplace at a very low rate. Since the reaction rate of the secondepoxy-based system should be very low, the second epoxy-based system mayadvantageously be catalysed by a catalyst, which is non-active untilactivated. This activation may for example be by UV-light, by additionof a compound or by heating; it is preferred, however, that the catalystis activated by heating.

The resin may for example be distributed as discrete points, as randomor organised lines, continuous or non-continuous layers or areas or anycombination of these. Furthermore, the resin or additional resin may beinfused during processing.

Besides fibres and resin, composite members, the transition zone and—ifpresent—the transition member may for example comprise one or more offillers (e.g. a cheap inert material) and/or solvents and/or diluentsand/or rheological agents and/or viscosity adjusting agents.

Traditionally, gas enclosed in the pre-form prior to and during curinghas been removed in the direction of the fibres, i.e. in the plane of aresin layer. Hence, the larger the structure, the longer the gas has totravel to be released from the structure. The risk that gas becomestrapped inside a cured structure is hence increased with the size of thestructure. It appears that the problem with entrapped gas isparticularly pronounced when the reinforcing with unidirectional fibres.It may be speculated that this is due to the very close packing of thefibres, which may arise in some areas of a composite reinforced byunidirectional fibres. However, problems concerning entrapped gas mayalso be present in other types of fibre orientation e.g. biaxial orrandom orientations. In a preferred embodiment of the present inventionthe connections are therefore prepared to enhance removal of air fromwithin the connections in a direction substantially orthogonal to thesurface. This may for example be realised by having a non-continuouslayer of resin within the transition zone.

By gas is herein meant entrapped atmospheric air as well as gaseousproducts, byproducts and starting materials related to the preparationprocess.

The method according to the invention may be adapted to automatedprocessing. For example, in the production of a transition zone or atransition member, a robot may advantageously distribute layerscomprising fibres, resin and, optionally, adhesive. Automation isfacilitated by an at least partial immobilisation of fibres duringlaying, by an adhesive which will prevent or at least greatly reducedisturbance in the layers comprising fibres. Furthermore, when theadhesive is only applied to selected areas of the ground plan of thetransition zone or the transition member, time is saved compared todistribution of resin over the entire ground plan.

Resin systems may contain components, which may be irritant or harmfulwhen in contact with naked skin, if ingested or inhaled. Avoidance ofdirect contact is therefore highly desirable. Since the processesaccording to the invention are particularly suited for automation, theproducts and processes according to the invention represent asignificant improvement to the working environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the general conception of the angle between the overallplane of the interface between the transition zone and one of thecomposite members.

FIG. 2 shows a preferred embodiment of a connection according to theinvention having macro grading.

FIG. 3 shows a preferred embodiment of a connection according to theinvention having micro grading and details of a micro graded layer.

FIG. 4 shows a further preferred embodiment of a connection according tothe invention having interlaced layers comprising fibres.

FIG. 5 shows yet another preferred embodiment of a connection accordingto the invention.

FIG. 6 shows sketches of some other preferred embodiments of connectionshaving interlaced layers comprising fibres.

FIG. 7 shows a sketch of the stress distribution near the end of afibre.

DESCRIPTION OF THE DRAWINGS

All the figures are highly schematic and not necessarily to scale, andthey show only parts which are necessary in order to elucidate theinvention, other parts being omitted or merely suggested.

In some of the figures separate layers comprising fibres are indicated.To increase clarity the number of layers shown is limited, and in realconnections and composite members the number of layers may in some casesbe considerably higher like for example 10's or 100's. The distancebetween the layers, the thickness of layers and the angles are examplesof schematic parts.

The physical properties of fibre reinforced composites are to a largeextent dictated by the reinforcement fibres. This includes propertieslike the coefficient of thermal expansion, CTE, and Young's modulus.Hence, a thoroughly connection is for example relevant when a compositecomprising carbon fibres are connected to a composite reinforced byanother type of fibres, since the CTE of carbon fibres is very low andmay even be negative. However, the same type of connections may be usedfor strong connections between composites reinforced by other types offibres. The fibres may be any of the fibre types mentioned previously inthis description. A connection according to the invention may forexample be used for connecting a carbon fibre reinforced composite to aglass fibre reinforced composite.

The connection according to the invention is best appreciated when asignificant difference in properties exists between the compositemembers to be connected, e.g. a first and a second composite member. Forexample the difference in CTE of the composites may be greater than3×10⁻⁶° C.¹ or even greater than 5×10⁻⁶° C.¹ or the difference inYoung's modulus may be greater than 25% of the lower value for thecomposites or even greater than 100% of the lower value. In many casescomposites having even greater differences in properties may beconnected by a connection according to the present invention. However, aconnection according to the invention is also usable when thedifferences in the properties are smaller.

The transition zone should have a layered structure, i.e. compriselayers comprising fibres. The number of layers may vary considerablydependent on the design of the connection and the size and type ofcomposite members. In some cases only a few, for example 2, 3, 4, 6 or10 layers are used whereas in other cases a higher number, for example20, 30, 50, 100 or more layers are needed to obtain the desired qualityof the connection. The degree of laminated structure is often reducedduring processing. For example pre-consolidation of a transition zone ora transition member tends to homogenise the structure. The structure ofthe transition zone is layered to enhance controlled orientation of thefibres, to facilitate optimisation of the reduction of stress betweenthe composites as well as to facilitate ease of production like forexample enhance suitability for automation of the production.

The fibres may be provided in any desirable orientation in thetransition zone like for example unidirectional, biaxial or random.However, the fibres are preferably oriented to reduce the stress betweenthe transition zone and the composite members and/or reduce the stressbetween the composite members as well as to strengthen areas of thefinal structure which will be exposed to a higher stress during service.

The orientation of fibres may or may not be the same in all the layerscomprising fibres within the transition zone; however, in a preferredembodiment the fibres are oriented substantially the same way in all thelayers of fibres. One or more layers of fibres may for example beoriented in another manner than other layers, if a stress analysissuggests a multi-axial fibre orientation. In another preferredembodiment, layers comprising first type fibres are mainlyunidirectional and layers comprising second type fibres are mainlybiaxial. Another way to strengthen an area of the final structure whichwill be exposed to higher stress during service, is to increase theamount of fibres in that area.

The resin may for example be distributed as discrete points, as randomor organised lines (preferably at an angle to the fibre orientation),continuous or non-continuous layers or areas or any combination ofthese. In a preferred embodiment, gas may escape orthogonal to thedirection of the fibres through non-continuous layer of resin. Resin oradditional resin may, furthermore, be infused during processing.

The resin may be provided between two layers comprising fibre forinstance individual or groups of fibres, fibre tows, fibre tow-pregs,prepregs, semi-pregs, woven or non-woven fabrics or mats. This is thepreferred positioning of resin and when this positioning is used, it ishighly desirable that the resin is distributed in non-continuous layers.However, the resin may also be provided in contact with only one layercomprising fibres, i.e. at the top or at the bottom of the transitionzone or transition member. In this case, it is preferred to provide theresin at the bottom of the transition zone or transition member and theresin may be provided in a continuous layer as gas will not usually haveto escape through the resin layer. In a preferred embodiment, resin isonly provided at the top and/or the bottom of the transition zone ortransition member, i.e. only adhesive is provided between the layerscomprising fibres. In another preferred embodiment, resin is onlyprovided at the bottom of the transition zone or transition member, i.e.between the layers comprising fibres only adhesive is provided.

In a preferred embodiment, the resin is provided to form anon-continuous layer or layers, as this facilitates removal of gasduring a subsequent consolidation and/or curing of the transition zone,the transition member or the composite member. The resin is preferablysemi-solid and may stick to and/or at least partially immobilise fibresof one or more layers. In another preferred embodiment, the resin isdistributed to form a pattern of solid or semisolid particles, and theseparticles may for example be sprinkled over a layer of fibres.Alternatively, discrete resin or adhesive dots may for example be formedfrom a resin provided as a liquid. A liquid resin may also be providedas a line or several lines, which may form an oriented pattern, a randompattern or a combined pattern. A different approach to a non-continuouslayer of resin is a sheet of resin wherein a number of through-goingholes are provided. As it is obvious from these examples of resinpatterns, a person skilled in the art will be able to provide otherpatterns without departing from the inventive idea of the invention.

Examples of preferred embodiments of continuous layers of resin are

-   -   layers prepared by distribution of a liquid resin,    -   layers prepared by infusion of resin into or between layers        comprising fibres,    -   layers prepared from a sheet of solid resin,    -   some but not all prepregs, etc.

The adhesive should at least partially immobilise the fibres in contactwith the adhesive. The adhesive may be any type of adhesive. Theadhesive should be compatible to the resin, however, and is preferably aresin-type adhesive and more preferably the adhesive is related to theresin of the transition zone, the transition member or the compositemember in the sense that it comprises compatible chemistry. One way toensure compatibility between the resin and the adhesive is to usesubstantially the same composition. In a preferred embodiment, thecomposition of the adhesive is the same as the composition of theadhesive. It is within the scope of the invention to use more than onetype of adhesive in a transition zone, a transition member or acomposite member. For example some portions of adhesive may have thesame composition as the resin, whereas other portions may have adifferent composition.

The adhesive may in principle be provided in the same patterns as theresin, however, it is preferred to provide a less dense pattern for theadhesive to save time. It is important to keep in mind that the purposeof the adhesive is to ensure that the layers comprising fibres are atleast partially immobilised to facilitate fibre laying. Furthermore, theadhesive will often increase the mechanical strength and hence improvethe handleability of an unconsolidated and uncured pre-form relative tothe equivalent structure without adhesive by at least partially fixingadjacent layers of fibre to each other. One way to ensure a facilitationof the fibre laying is to provide a strip of adhesive close to orexactly where the layers comprising fibres are initiated during fibrelaying. Other embodiments are also feasible such as dots, broken orcurved lines, etc. In principle continuous layers may be used; however,the advantage of using an adhesive over using a resin will be reducedthereby. In some cases, automation may favour an adhesive pattern, wherethe adhesive is applied in a continuous line, for example as a zigzag orcriss-cross pattern. A person skilled in the art will appreciate theadvantage of providing only a limited amount of adhesive compared toeither a full or nearly full layer of resin or cross-ply stitching,particularly with respect to the time saved during processing and easeof automation.

Three-dimensional forming may be applied for reducing the interfacialstress in a connection between composite members. A three-dimensionalform may for example be realised by selective initiating and/orterminating of for example fibre tows or other types of layerscomprising fibres during fibre laying. Traditionally, suchthree-dimensional forms will involve the creation of a large contactarea orthogonal to the main stress direction in at least one axis. InFIG. 1 it is observed that the contact area may be increased bydecreasing the angle α between the overall plane of the interface 2between the transition zone and a composite member 10 and a surface 4 ofthe composite member 12. A distance much greater than the orthogonaldistance between adjacent layers comprising fibres separates theterminations of the layers of fibres in the tapered section (2, 4),which will tend to reduce the interfacial stress. Furthermore, if theangle α is sufficiently low, the end of a layer comprising fibres of thefirst composite member may be placed over an adjacent layer comprisingfibres from the second composite member in the transition zone. This maylead to a side-by-side coupling of the fibre layers, which is favourablecompared to an end to end coupling.

The fibres in a composite member near the transition zone may forexample be oriented substantially parallel to the overall plane of theinterface between the transition zone and that composite member, thefibres may be oriented substantially parallel to a surface of thatcomposite member, the fibres may be oriented in a combination of theseorientations, the fibres may be provided with a random distribution,etc. It is preferred, however, that the fibres in the composite membersnear the transition zone are substantially parallel to a surface of thecomposite member as this may provide an—even if limited—side-by-sidecoupling between the fibres. This is particularly the case when theangle α is low. Preferably the angle α should be less than about 10° butan even better connection may by obtained when the angle is less thanabout 2°. If the utilised fibres are very stiff such as carbon fibres amay in some cases advantageous be as low as 0.5° to 1° or even lower.

If thick composites are to be connected, low angles may be difficult torealise as this will require an unreasonable long transition zone. Insuch situations it may be advantageous to use a zigzag interface or touse a number of connections having the angle α on top of each other.This may for example be realised by separating the thick compositemembers into a number of thinner composite members or flanges and thenconnect the thinner pairs of composite member or flanges individually ontop of each other or near each other.

The transition zone may be prepared in several ways. In a preferredembodiment, the transition zone comprises a transition member. Thetransition member is composite material comprising layers comprisingfibres and a resin. In a preferred embodiment the transition member isprepared independently of the composite members to be connected. In thisembodiment the transition member may for example be prepared as apre-form and optionally be pre-consolidated prior to the connection tothe composite members. A method for preparing a connection involving atransition member may comprise the steps of:

-   -   providing at least one layer comprising first type fibres for        the transition member    -   providing at least one layer comprising second type fibres for        the transition member    -   providing a resin in contact with at least one of the layers        comprising fibres for the transition member    -   connecting the transition member to the first composite member    -   connecting the transition member to the second composite member    -   curing the transition member, and    -   optionally co-curing the first and/or the second composite        member with the curing of the transition member.

The layers comprising first type fibres or second type fibres,respectively, may be the same layer or layers if the fibre types aremixed. The resin may or may not have the same composition as one of thecomposite members, however, it is preferred that the resin is compatiblewith the resin of the composite members and more preferably the resincomposition is substantially the same as the composition of one or bothof the composite members. The resin is preferably but not necessarilynon-continuous in the sense that gas may be removed from the transitionmember orthogonal to a layer of resin. The transition member may forexample also comprise an adhesive between two or more of the layerscomprising fibres if this is desirable. One or more of the compositemembers may be co-cured together with the curing of the transitionmember. In a preferred embodiment, the complete structure is co-cured inone operation.

In a preferred embodiment, the transition member is pre-consolidated,preferably prior to the connection of the composite members.Pre-consolidation of the transition member is particularly advantageouswhen at least one of the composite members to be connected is curedprior to forming of the connection, as pre-consolidation may reduceshrinkage during curing.

In a preferred embodiment an uncured or pre-consolidated transitionmember may be transported or stored for a prolonged period of time suchas weeks or months without premature curing.

The layers of the layered structure of the transition zone are typicallyoriented substantially parallel to the overall plane of the interfacebetween the transition zone and at least one of the composite members orthe layers are oriented substantially parallel to a surface of at leastone of the composite members. In some embodiments both options arepossible and hence the design should be chosen depending on the desiredproperties of the final structure and the types of composite membersinvolved.

One approach to the designing the transition zone is to provide agradual change of the composition of the layers comprising fibres fromthe composition of the first composite member to the composition of thesecond composite member. In FIG. 2 this gradual change is realised bymacro-grading. By macro-grading is meant that the transition zonecomprises a number of layers each comprising fibres having eithersubstantially the same fibre composition 14 as the first compositemember 10 or substantially the same fibre composition 16 as the secondcomposite member 12. In a preferred embodiment a first side of a stackof such layers is connected to the first composite member 10 and asecond side of the stack is connected to the second composite member 12.Hence, the layers are typically oriented parallel to the overall planeof the interface between the transition zone and at least one of thecomposite members.

The transition zone shown in FIG. 2 are preferably prepared directlyonto one or both of the composite members, but other routes ofpreparation are feasible for example the route involving preparation ofa separate transition member. The interface between the first compositemember and the transition zone need not be parallel to the interfacebetween the second composite member and the transition zone, however ina preferred embodiment the interfaces are substantially parallel sincethis provides for an easier manufacturing.

The gradual change may for example be realised by changing the frequencyof the two types of layers comprising fibres for example as it is shownin FIG. 2. Other examples are sequences like:

Bulk A-B-A-Bulk B Bulk A-B-A-A-B-A-B-B-A-Bulk B BulkA-B-A-A-A-B-A-A-B-A-B-B-A-B-B-B-A-Bulk B

Here A indicates a layer comprising fibres having substantially the samefibre composition as the first composite member, B indicates a layercomprising fibres having substantially the same fibre composition as thesecond composite member and Bulk indicates the composite member.

The sequence need not be symmetrical and the optimum sequences should bechosen with respect to properties, layer thickness, etc. in theparticular situation. The examples of sequences is by no meansexhausting for useful sequences and a person skilled in the art will beable to provide a variety of sequences without departing from theconception of the inventive idea.

When the transition zone is prepared directly on one or more compositemembers the connection may be prepared by these steps:

-   -   providing a first composite member    -   providing at least one layer comprising first type fibres on or        in connection to the first composite member or on a previous        layer comprising fibres in the transition zone    -   providing at least one layer comprising second type fibres on or        in connection to the first composite member or on a previous        layer comprising fibres in the transition zone    -   providing a resin in contact with at least one of the layers        comprising fibres in the transition zone    -   connecting the structure to the second composite member    -   curing the transition zone, and    -   optionally co-curing the first and/or the second composite        member together with the with the curing of the transition zone.

The transition zone may be prepared directly on both the first and thesecond composite member simultaneously if this is desirable.

A method for preparing connections comprising interlaced layerscomprising fibre may for example comprise these steps:

-   -   providing layers comprising first type fibres    -   providing a means for at least partially immobilising the        fibres, said mean for example comprising an adhesive or a resin    -   providing layers comprising second type fibres to be at least        partially interlaced between layers comprising first type        fibres, said layers comprising second type fibres extend beyond        the first composite member    -   providing a resin in contact with at least one of the layers        comprising fibres    -   connecting the structure to the second composite member    -   curing the transition zone and the first composite member, and    -   optionally co-curing the second composite member together with        the with the curing of the transition zone.

In a similar way the transition zone may be partially incorporated intothe second composite member.

The transition zone may be pre-consolidated prior to the curingoptionally the pre-consolidation is conducted on the transition zone andat least one of the composite members simultaneously.

The fibres for the layers comprising fibres may for example be providedas prepregs, semi-pregs, woven or non-woven fabrics, mats, pre-forms,pre-consolidated pre-forms, individual or groups of fibres, tows,tow-pregs or a combination of these.

Another approach to gradually changing the composition of the layerscomprising fibres involves micro-grading. By micro-grading is meant thatthe first type fibres and the second type fibres are mixed in at leastone layer in the transition zone comprising fibres. Alternatively, alayer comprising fibres or a mixture of fibre types having intermediateproperties of first type fibres and second type fibres is present;Particularly properties critical to the connection (e.g. CTE and Young'smodulus) should be intermediate to those of the first type fibres andthe second type fibres. An example of micro grading is shown in FIG. 3Awhere the connection has two micro-graded layers 20 a and 20 b betweenthe first composite member 10 and the second composite member 12. As itis indicated with the relative shading of 10, 20 a, 20 b and 12 in FIG.3A it is preferred that the ratio of first type fibres to the secondtype fibres decreases gradually from the first composite member towardsthe second composite member. The micro-graded members 20, 20 a and 20 bmay for example be prepared from a thick fibre mat comprising ahomogeneous mixture of first type fibres and second type fibres and aresin, but the person skilled in the art will know other ways to realisea micro-graded material. The micro-graded members may for example beprepared from a number of thinner layers comprising fibres. Thesethinner layers being shown in FIGS. 3B and C as dark lines 22. Theselayers may generally be oriented parallel to a surface of the finalstructure as indicated in FIG. 3B or parallel to a larger surface of themicro-graded member as indicated in FIG. 3C. It is also within the scopeof the invention for example to prepare the transition zone or atransition member directly from thinner micro graded layers comprisingfor example one layer of fibres, one layer of fibre tows or a prepreg.

In a preferred embodiment of a micro graded connection, the fibreswithin a micro graded layer comprising a mixture of first type fibresand second type fibres are distributed in a homogeneous ornon-homogeneous manner within an individual layer. In FIG. 3D an exampleof a homogeneous distribution is shown. It is observed that theconcentration of tows 15 of first type fibres and tows 17 of second typefibres is constant. The orientation of the first and the second typefibres need not be the same even it this is shown in FIG. 3D. In FIG. 3Ean example of a non-homogeneous distribution of fibre tows is shown. Itis observed that the concentration of tows 15 of first type fibresrelative to tows 17 of second type fibres is much greater near the edgethan near the centre of the layer. Other sources for fibres may equallywell be used instead of or together with fibre tows. In non-homogeneouslayers, the concentration and/or the orientation of the first typefibres are preferably different from those of the second type fibres,but other examples of inhomogeneous manners may be provided by a personskilled in the art in the light of these examples. This type of layersmay for example be advantageous if more than two composite members areto be connected or if at least one of the composite members isinhomogeneous.

FIG. 4 shows a transition zone comprising two types of layers or sheetscomprising fibres, one type 50 comprising fibres having substantiallythe same fibre composition as the first composite member 10 and anothertype 52 comprising fibres having substantially the same fibrecomposition as the second composite member 12. The two types of layersare partially interlaced in the sense that in a part of the transitionzone at least one of the types of layers extend beyond the other type oflayer. At least some of this part of the transition zone is used for theconnection with the composite member having substantially the same fibrecomposition. Due to the interlaced layers the contact surface is veryhigh and the connection is hence under the right conditions very strong.In a preferred embodiment both types of layers extend beyond the otherin a part of the contact zone as shown in FIG. 4.

In a preferred embodiment the transition zone is integrated at leastpartially with one of the composite members or a pre-form. In FIG. 5 anexample of such an embodiment is shown. The transition zone may forexample comprise layers 62 comprising second type fibres initiatedwithin the first composite member being reinforced by layers 60comprising first type fibres and extending beyond the first compositemember. The distance 64 from the ends of layers comprising first typefibres to the adjacent ends of layers comprising second type fibresshould preferably be great enough to prevent or reduce coupling ofstress between the layers (see below). It is also preferred that thedistance 66 between the ends of adjacent layers comprising first typefibres is great enough to prevent or reduce coupling of stress betweenthe layers. The transition zone having interlaced layers comprisingsecond type fibres is preferably prepared as a part of the preparationof the first composite. In a preferred embodiment, the layers comprisingsecond type fibres are provided as prepregs. The prepregs may beunidirectional prepregs, however, experimental results suggest,surprisingly, that biaxial prepregs comprising the second type fibresprovide a better basis for connecting of the pre-form to a structurereinforced by second type fibres. It may be theorised that this is dueto better coupling between layers where the relative main fibreorientation is not parallel, however, other effects may take part inthis.

Experimental results have shown that the embodiments described inrelation to FIG. 4 and FIG. 5 are capable of successfully connecting aglass fibre reinforced base of a spar for a wind turbine blade to acarbon fibre flange reinforced spar. The success in this highperformance application indicates the span of the present invention.

In a preferred embodiment of a connection, the transition zone compriseslayers, which comprise fibres, extending from the first composite memberinto the second composite member and/or layers, which comprise fibres,extending from the second composite member into the first compositemember. This preferred embodiment may for example be established in oneoperation. More preferred embodiments having interlaced layerscomprising fibres are shown in FIG. 6. FIG. 6A shows a connection wherethe layers of each composite member comprising fibres extend into theother composite members 10 and 12. This type of connection is typicallyprepared simultaneously with the fibre laying, i.e. preparation of bothcomposite members and the transition zone in one operation. In FIGS. 6Aand B all layers comprising fibres extend into the other compositemember, however, this need not be the case unless it is required toachieve the desired strength. For example the transition zone may becomethicker if all layers are continued into the other composite, whichagain may lead to bending of fibres entering into the transition zone.In FIG. 6B the transition zone 13 has been indicated by a broken line asan example of the extent of a transition zone.

FIG. 6B shows an integrated connection between two composite membershaving an interface which is substantially orthogonal to the surface ofthe composite members. To reduce the effect of the stress end-condition(see below), the ends of the fibre layers are provided in a zigzagpattern.

In a preferred embodiment of the connection shown in FIG. 6B, a zigzagpattern is also provided in the dimension of the interface orthogonal tothe plane shown in the figure. In other words, the line defined by theend of a layer comprising fibres is not a straight line in thispreferred embodiment.

FIG. 6C shows an example where the composite member 10 is prepared forconnection with composite member 12 in that layers 40 havingsubstantially the same fibre composition as composite member 12 has beeninterlaced into composite member 10. In FIG. 6D the connection betweenthe composite members is formed. It is observed that the layers 40 arebeing bent to form the layers 42. The bending is not desirable, but thebending angle may be greatly reduced by decreasing the angle α, whichhas been defined in relation to FIG. 1. In a preferred embodiment, α istypically kept below 10° as described elsewhere. The layers 42 may ormay not comprise a resin (e.g. a prepreg or applied directly), butduring curing of the transition zone a resin should be present to ensureformation of a cured matrix of resin.

A method for preparing a connection as shown in FIGS. 6A and B whereinthe transition zone comprises layers comprising fibres extending fromthe first composite member into the second composite member and/orlayers comprising fibres extending from the second composite member intothe first composite member, may for example comprise the steps of:

-   -   providing layers comprising first type fibres, at least some of        these layers extend into a part of the structure resembling the        second composite member    -   providing layers comprising second type fibres, at least some of        these layers extend into a part of the structure resembling the        first composite member    -   optionally providing a means for at least partially immobilising        the fibres, said means for example comprising an adhesive or a        resin    -   providing a resin in contact with at least one of the layers        comprising fibres    -   optionally pre-consolidating the structure, and    -   curing the structure

In a preferred embodiment, the structure is co-cured altogether,however, curing or pre-consolidating may be carried out on selectedparts of the structure in individual operations.

When a fibre reinforced material is stressed by a force F, the matrixmaterial, i.e. the resin, is stressed equally along the layerscomprising fibres. However, if a layer comprising fibres is stresseddifferently for example as it is the case for interlaced, non-continuouslayers comprising fibres stress may build up in the resin. In FIG. 7B aschematic illustration of the shear stress in the resin between fourlayers comprising fibres are shown. The layers 30 a, 30 b and 30 dcomprising fibres are continuous layers such as continuous layerscomprising second type fibres in the second composite member whereas thelayer 30 c comprising fibres is a non-continuous layer such as aninterlaced layer comprising first type fibres interlaced from a firstcomposite member into a second composite member. The mainly verticallines represent the local shear stress in the resin, i.e. a verticalline indicates that the resin experiences the average shear stress andan angled line indicates a difference in the experienced shear stressfrom the average resin. It is observed that a large local difference inthe shear stress is present near the end of the layers 30 c.

In FIG. 7A the shear stress build up near the end of layer 30 c isrepresented as a schematic plot of the variation in the shear stress inthe resin along a line parallel to and near layer 30 c relative to theaverage shear stress in the resin. It is observed that difference inshear stress is greatest at the end of the layer 30 c. Furthermore, itis observed that at a certain distance away from the end of layer 30 cthe shear stress is about the average level of shear stress. When endsof layers comprising fibres are provided closer to each other than theextent 34 of the end-condition of the shear stress, the shear stress maycouple between the layers and hence form a weak point, line or planewithin the structure. To reduce or prevent coupling of shear stress fromthe end of one layer to the end of a nearby layer, the distance betweenthe ends of interlaced layers should be greater that the extent of theend-condition. Since the extent of the end-condition is difficult toestablish, it is preferred to use a safety margin and hence separate theends of two adjacent layers by at least two times the extent of theend-condition.

It is also reasonable to ensure that the distance between the nearestlayer end of the same type of fibre should be separated by a distancecorresponding to the extent of the end-condition, preferably with asafety margin and hence using a factor of two.

The actual extent of the shear stress concentration depends on a numberof factors, such as thickness of layers comprising fibres and layerscomprising primarily resin, the type of fibres, the type of resin, etc.The extent of the stress concentration may for example be established bymodelling or by empirical methods.

When larger structures comprising pre-forms or composite members are tobe prepared, this may follow a method wherein the not fully curedmaterial is shaped at least partially plastically. The pre-form may beconnected to further pre-forms before or after shaping to provide alarger structure. The pre-form may also be connected to otherstructures. It is preferred but not required that the connectionsinvolve a tapered part or layers comprising second type fibres. When thematerials have substantially the same properties it may be sufficient touse a tapered section as shown in FIG. 1 to realise a reasonableconnection, however, the greater the difference in properties thegreater care should be taken.

Relevant curing processes and processing parameters are known to theperson skilled in the art, however, it is preferred that a pressure isapplied on the structure and/or a vacuum is applied to the structure aspart of the curing procedure. The pressure may be applied via a vacuumin a vacuum enclosure surrounding all or part of the structure. It ispreferred that the pressure and/or vacuum is applied on the structurebefore the curing reaction is commenced as this facilitate the removalof gas from the structure.

The connections and particularly separately prepared transition membersas discussed previously may advantageously be pre-consolidated toprovide a more homogeneous material with better performance. Transitionmembers may often be considered as pre-forms.

In a preferred embodiment, the transition member or the transition zone(in the following sections referred to as the element) is treated bypre-consolidation to form a pre-consolidated element as described in thefollowing section. Pre-consolidation is particularly useful when thefibres are provided as individual or groups of fibres, fibre tows orfibre tow-pregs compared to fibres provided in prepregs as a lowerviscosity may be realised during the pre-consolidation process. Thiswill increase the redistribution of resin and/or fibres, which is highlydesirable as it increases the homogeneity of the resulting product.However, pre-consolidation of elements comprising fibres provided inprepregs, semi-pregs, woven or non-woven fabrics and/or mats may also beadvantageous.

By pre-consolidation is herein meant a process, whereby gas inside anelement is removed and a low porosity element is produced.Pre-consolidation involves redistribution of a resin and optionally aredistribution of fibres. Furthermore, pre-consolidation may involve alimited curing of the resin. Pre-consolidation is particularly useful asit produces a dense element (hereinafter named a pre-consolidatedelement). Pre-consolidated elements and composites prepared frompre-consolidated elements will be appreciated amongst others due to goodreproducibility, low porosity, high homogeneity, high strength, abilityto plastically shaping, ability to be connected to other elements and/orother structures, suitability for automation and long shelf life withoutpremature curing.

When the pre-consolidation involves a limited curing, this limitedcuring may involve a release of up to 50% of the energy that will bereleased by a complete curing of the resin. However, it is preferredthat the extent of curing is limited to an extent that will allow thepre-consolidated element to be deformed plastically. The degree ofcuring that will allow for plastical deformation of a pre-consolidatedelement depends amongst others on the resin as well as on the fibre typeand fibre content. Generally, it is preferred that the limited curinginvolves less than about 20% of the energy that will be released by acomplete curing of the resin and more preferably that the limited curinginvolves between 3 to 15% of the energy that will be released by acomplete curing.

Generally speaking, the pre-consolidation process should reduce theporosity of the element and it is preferred that the resulting porosityof the pre-consolidated element is less than 5% by volume, preferablyless than 2% by volume and more preferably less than 1% by volume. Insome cases, a porosity of even 1% may reduce the properties of acomposite considerably. In these cases, it will be appreciated that themethod and the pre-consolidated element may be produced with porositieswell below 1%. For example, a reproduced porosity of about 0.2% byvolume was realised for a composite with 60% carbon fibres in epoxy. Thereduction of the porosity may for example be a result of exposing theelement to a pressure and/or a vacuum in relation to thepre-consolidation process.

The porosity of the pre-consolidated element can not be establisheddirectly, as a density is not known and may vary throughout thematerial. Hence, the porosity should be established by an optical methodon a materialographic sample. Preparation of materialographic samplesfrom an uncured pre-consolidated element is very demanding, since thematerial comprises both a very soft element (i.e. a resin) and a veryhard element (i.e. the fibre). To establish a reproducible result, it ishence necessary to cure the element prior to materialographicpreparation. This curing should be pressureless to ensure that theporosity is unaffected by the process.

To ensure handleability, the pre-consolidated element should besubstantially unsticky, i.e. it should be easily releasable from anyrelevant surface and it should not leave excessive amounts of resin on asurface when released.

To ensure a long shelf life and/or stability during transportation it isimportant that the rate of the curing reaction of the bulk of the resinis sufficiently low at room temperature and that a catalyst—ifpresent—is not activated by accident. For example, if the catalyst isactivated by heating, it should be ensured that the activationtemperature is considerably higher than the expected maximum temperatureduring storage.

In a preferred embodiment, the pre-consolidated elements are at leastpartially deformable. This may for example be realised through abalanced and limited curing during the pre-consolidation process. In apreferred embodiment, at least a part of a pre-consolidated element iscapable of being bent around an axis parallel to the main fibreorientation with a diameter of more than 1 cm, however, in some cases apre-consolidated element may be bent with a diameter of more than 5 cmby plastic deformation. The low bending diameters may be realised byrearranging of resin and/or fibres or by three-dimensional forming of anelement. By three-dimensional forming is herein meant that the thickness(e.g. the number of layers or amount of fibres and/or resin) and/or theform of the ground plan is adjusted for a part of the element relativeto the bulk of the element. Typically, only a part of thepre-consolidated element is prepared for very sharp bending, whereasbending around an axis with larger diameters, e.g. 50 cm, may often berealised by all parts of the pre-consolidated element.

The stiffness of a element realised during a pre-consolidation processshould ensure that the pre-consolidated element is sufficiently stiff toprevent relaxation of the pre-consolidated element in the lengthdirection of the fibres when placed on a non-flat surface and yet allowfor plastic deformation around an axis parallel to the longitudinaldirection of the fibres.

The pre-consolidation process often leads to an increase in viscosity ofthe resin in the element, for example by a partial curing. It ispreferred that the viscosity at room temperature is increased by afactor of at least two and more preferably by a factor of at least five,as an increase in viscosity will enhance handleability, strength andunstickyness. In some cases, the viscosity may be increased by a muchhigher factor of for example 10, 100 or 1000. This is for example thecase if part of the resin is injected into the element as a roomtemperature liquid. Another way to express the increase in viscosity isto look at viscosity directly. It is preferred that the viscosity of theresin in the unconsolidated element is between about 10 to 10,000 cP atthe temperature where the pre-consolidation process is conducted,preferably between about 500 to 5,000 cP.

The temperature where the pre-consolidation process is conducted mayvary considerably depending particularly on the composition of theresin. Typically, the pre-consolidation temperatures for epoxy-basedresin systems are 50 to 90° C. and preferably 60 to 80° C., however,both higher and lower temperatures may be feasible in some systems.

The pre-consolidation process may lead to an increase in the glasstransition temperature, T_(g), of the resin, for example by a partialcuring. It is preferred that the T_(g) of the resin is increased duringpre-consolidation by at least 2° C. and preferably by at least 5° C., asan increase in T_(g) usually indicates an increase in the averagemolecular weight of the resin, which will enhance handleability,strength and unstickyness. In some cases, T_(g) may be increased more.This is particularly the case when T_(g) of the unconsolidated elementis very low.

In a preferred embodiment a pre-consolidated element according to theinvention with an epoxy-based resin system should typically have a T_(g)between −10 to +30° C. and preferably a T_(g) between −5 to 10° C. Insome cases, T_(g) of the resin of the pre-consolidated element is higherthan about 0° C. and preferably higher than about 3° C. For theunconsolidated element, T_(g) of the resin should be below about 5° C.and preferably below about 2° C.

In some cases, curing of a pre-consolidated element without beingexposed to a vacuum will result in a material with properties equivalentto a vacuum-cured element, since porosity has been eliminated or greatlyreduced during the pre-consolidation process prior to the curing. Thismay for example be used when preparing a connection in a large structurewhere the establishing of a vacuum may be very time-consuming.

In a preferred method of pre-consolidating and/or curing an element, theelement is placed on a reactor surface like for example a plate, amould, etc. It is preferred that the reactor surface is flat and that itwill withstand heating and/or vacuum. Then a pressure is applied to theelement. The pressure may be applied by a press or—preferably—via avacuum within a vacuum enclosure. If a vacuum is used, then a vacuumenclosure should be obtained prior to pressing. The vacuum enclosure mayfor example comprise a vacuum bag or it may comprise a reactor surfaceand a flexible cover connected in a vacuum-tight way to the reactorsurface. Gas may for example be evacuated through the reactor surface orthrough an opening in the vacuum bag or flexible cover. Thenpre-consolidation or curing is activated. The activation may take placebefore and/or during and/or after applying pressure. The activationcomprises a reduction of the viscosity of the resin. This may forexample be realised by physical means (e.g. heating, addition ofsolvent, pressure etc.) and/or by a chemical reaction. During thepre-consolidation process, a limited curing may or may not take place.When the porosity has been reduced to a desired level or another objectof the pre-consolidation is obtained, the pre-consolidation process isterminated. The termination may for example be a result of exhaustion ofa first resin system or cooling of the pre-consolidated element to atemperature, where the curing reaction is sufficiently slow and/or theviscosity is sufficiently high for the pre-consolidated element toachieve the stability needed for the desired shelf life.

In a preferred embodiment, the element to be pre-consolidated or curedis having at least one non-continuous layer of resin, through which gasmay be removed during the pre-consolidation or curing process. Hence,the gas need not be removed from the element via one plane of a layer ofresin or in a plane of one layer comprising fibres. Thus, thetransportation distance and risk of having trapped gas inside thepre-consolidated element is greatly reduced. In a more preferredembodiment, all layers of resin—optionally except from a layer on top ofthe top layer comprising fibres or below the bottom layer comprisingfibres—are non-continuous.

An example of a method for ensuring that gas may continuously be removedfrom the element during pre-consolidation or curing involves a gradualactivation of the pre-consolidation process starting either from thecentre of the element and moving towards the surfaces or from a side oredge and moving through the element. For example this may be realised byheating from the reaction surface only, thereby activating thepre-consolidation process gradually from the side of the element incontact with the reaction surface or by controlled microwave heating,thereby activating the pre-consolidation process gradually from theinside of the element and moving towards the surfaces.

The properties of a laminated structure having layers comprisingoriented fibres to a large extent depend on the distribution of the mainelements of the structure i.e. resin, fibres and porosity. The resinpossesses a low strength compared to the fibres and may provide a routefor crack propagation through the structure, if too thick layers ofresin are present. Porosity may reduce the strength of the structuredramatically but the adversity depends on the size of pores, the shapeand the distribution, e.g. the effect of small, isolated spherical poresis limited, whereas larger pores positioned in the interface betweenresin and fibres may be fatal to the structure. Consequently it is vitalto be able to control the distribution of the elements.

The extent of redistribution or homogenisation during pre-consolidationor curing depends on the viscosity of the resin during the compactionprocess, i.e. the lower the viscosity the easier the redistribution ofthe elements. By utilising a pre-consolidation process the viscosity ofthe resin may be lowered more than what is feasible in the prior art,since the structure is not limited to supporting a particular shapeduring the process. When the pre-consolidation involves a limited curingof the resin, the viscosity may be further reduced since the limitedcuring increases the handleability and reduces the sticking of thepre-consolidated element. Hence, pre-consolidation typically allows forredistribution of resin and/or fibres to a greater extent than what maybe realised by direct curing. The resulting pre-consolidated elementsmay possess very low porosity as well as a more homogeneous structure.This may for example result in a composite structure having a lesspronounced laminated structure, i.e. where the layers are lesspronounced than a corresponding composite structure comprising onlyelements that were not pre-consolidated prior to curing.

Connections and methods according to the invention are useful for theconnecting of two relatively incompatible composite members whereascomposite members having compatible properties may also be connectedusing a connection or a method according to the invention. The extent ofthe invention is best realised when trying to connect two incompatiblecomposite members. Examples of relatively incompatible composite membersare composite members having a considerable difference in propertiessuch as coefficient of thermal expansion and/or Young's modulus, etc.

Connections and methods according to the invention are particularlyuseful for preparation of wind turbine blades. For example in thepreparation of a spar for a wind turbine blade, the connections may beused for connecting a mainly glass fibre reinforced part, e.g. the base,to a mainly carbon fibre reinforced part of the spar, e.g. the mainpart. This is particularly useful for larger blades as the weight of ablade may be dramatically reduced, when part of the glass fibrereinforced composite is replaced by carbon fibre reinforced composite.

Table for identification  2 Overall plane of the interface between thetransition zone and a composite member  4 Surface of a composite memberα Angle 10 Composite member 12 Composite member 13 Transition zone 14Layer comprising fibres having substantially the same com- position ascomposite member 10 15 Tow of first type fibres 16 Layer comprisingfibres having substantially the same com- position as composite member12 17 Tow of second type fibres 20 Layer having micro grading 20a Layerhaving micro grading 20b Layer having micro grading 22 Layer comprisingfibres F Force 30 Layer comprising fibre 30a, b, d Continuous layercomprising fibres 30c Non-continuous layer comprising fibres 32 Linesindicating difference in the experienced stress from the average resin34 The extent of the end-condition of the stress 40 Layers havingsubstantially the same fibre composition as a composite member 42 Layershaving ssubstantially the same fibre composition as a composite memberand being bend to follow the shape of the interface 50 Sheet or layercomprising first type fibres 52 Sheet or layer comprising second typefibres 60 Layers comprising first type fibres 62 Layers comprisingsecond type fibres 64 Distance between layers comprising first typefibres and adjacent layers comprising second type fibres 66 Distancebetween adjacent layers comprising second type fibres

The invention claimed is:
 1. A method for connecting a first compositemember comprising first fibres composed of a first material and a firstresin to a second composite member comprising a second resin and secondfibres composed of a second material different than the first material,using a connection comprising a layered structure comprising atransition member, the method comprising: assembling the transitionmember, which includes: providing at least two first layers composed ofthe first fibres, the first fibres being fully oriented in each firstlayer; providing at least two second layers composed of the secondfibres, the second fibres being fully oriented in each second layer;interlacing the at least two first layers and the at least two secondlayers into partially overlapping relation such that at least one of thefirst layers is positioned adjacent to and between two second layers andextends in its plane beyond the two adjacent second layers, and at leastone of the second layers is positioned adjacent to and between two firstlayers and extends in its plane beyond the two adjacent first layers;providing a resin in contact with at least one of the first or secondlayers to form the transition member; connecting the transition memberto the first composite member; connecting the transition member to thesecond composite member; and curing the transition member.
 2. The methodof claim 1, further comprising co-curing the first and/or the secondcomposite member with the curing of the transition member.
 3. The methodof claim 1, further comprising pre-consolidating the transition member.4. The method of claim 1, wherein the first composite member and thesecond composite member are joined together, substantially end-to-end,via the transition member, in a direction of respective longitudinalplanes of the first layer and the second layer.
 5. The method of claim1, wherein the first resin, the second resin, and the resin in contactwith at least one of the first or second layers are composed of athermosetting resin, and wherein the first material has at least onephysical property such as a coefficient of thermal expansion or Young'smodulus that is different than the same physical property of the secondmaterial.
 6. A method for connecting a first composite member comprisingfirst fibres composed of a first material and a first resin to a secondcomposite member comprising a second resin and second fibres composed ofa second material different than the first material, via a connectioncomprising a layered structure which forms a transition zone, thetransition zone comprising respective layers of the first fibres and thesecond fibres, the method comprising: providing the first compositemember; providing the second composite member; assembling the transitionzone, which includes: providing at least two first layers for formingpart of a layered structure and comprising first layer fibres havingsubstantially the same composition as the first fibres, the first layerfibres being one of: (i) the first fibres of the first composite member,(ii) connected to the first composite member, and (iii) on a previouslayer comprising fibres which form part of the layered structure in thetransition zone; providing at least two second layers, which, with theat least two first layers, forms the layered structure of the transitionzone, the at least two second layers comprising second layer fibreshaving substantially the same composition as the second fibers, thesecond layer fibres being one of: (i) part of the first compositemember, (ii) connected to the first composite member, and (iii) on aprevious layer of the layered structure of the transition zone,interlacing the at least two first layers and the at least two secondlayers into partially overlapping relation such that at least one of thefirst layers is positioned adjacent to and between two second layers andextends in its plane beyond the two adjacent second layers, and at leastone of the second layers is positioned adjacent to and between two firstlayers and extends in its plane beyond the two adjacent first layers;and providing a resin in contact with at least one of the first orsecond layers comprising fibres in the layered structure; connecting thelayered structure to the second composite member; and curing thetransition zone.
 7. The method of claim 6, further comprising co-curingthe first and/or the second composite members together with the curingof the transition zone.
 8. The method of claim 6, wherein the firstcomposite member and the second composite member are joined together,substantially end-to-end, via the transition zone, in a direction ofrespective longitudinal planes of the first layer and the second layer.9. The method of claim 6, wherein the first resin, the second resin, andthe resin in contact with at least one of the first or second layers arecomposed of a thermosetting resin, and wherein the first material has atleast one physical property such as a coefficient of thermal expansionor Young's modulus that is different than the same physical property ofthe second material.
 10. A method for connecting a first compositemember comprising first fibres composed of a first material and a firstresin to a second composite member comprising a second resin and secondfibres composed of a second material different than the first material,via a connection comprising a layered structure which forms a transitionzone, wherein the transition zone is integrated at least partially withthe first composite member, the method comprising: providing a firstcomposite member comprising first layers, wherein at least two of thefirst layers extends into a transition zone and includes first fibresthat are fully oriented with each other; providing at least two secondlayers extending into, the transition zone, and including second fibresthat are fully oriented with each other, interlacing the at least twofirst layers and the at least two second layers into partiallyoverlapping relation in the transition zone to form a layered structuresuch that at least one of the first layers is positioned adjacent to andbetween two second layers and extends in its plane beyond the twoadjacent second layers, and at least one of the second layers ispositioned adjacent to and between two first layers and extends in itsplane beyond the two adjacent first layers; providing a resin in contactwith at least one of the first and second layers; connecting the firstcomposite member to the second composite member; and curing thetransition zone and the first composite member.
 11. The method of claim10, further comprising co-curing the second composite member togetherwith the curing of the transition zone.
 12. The method of claim 11,further comprising pre-consolidating the transition zone.
 13. The methodof claim 12, further comprising pre-consolidating the first compositemember.
 14. The method of claim 10, further comprising providing a meansfor at least partially immobilizing the fibres.
 15. The method of claim10, wherein connecting the first composite member to the secondcomposite member comprises connecting the second composite member to thesecond layers of the transition zone.
 16. The method of claim 10,wherein the first composite member and the second composite member arejoined together, substantially end-to-end, via the transition zone, in adirection of respective longitudinal planes of the first layer and thesecond layer.
 17. The method of claim 10, wherein the first resin, thesecond resin, and the resin in contact with at least one of the first orsecond layers are composed of a thermosetting resin, and wherein thefirst material has at least one physical property such as a coefficientof thermal expansion or Young's modulus that is different than the samephysical property of the second material.
 18. A method for connecting afirst composite member comprising first fibres composed of a firstmaterial and a first resin to a second composite member comprising asecond resin and second fibres composed of a second material differentthan the first material, via a connection comprising a layered structurewhich forms a transition zone, the method comprising: providing at leasttwo first layers comprising first fibres and extending outward from thefirst composite member to form a first composite member extendingstructure that extends into the second composite member; providing atleast two second layers comprising second fibres and extending outwardfrom the second composite member to form a second composite memberextending structure, the second composite member extending structurebeing complementary to the first composite member extending structure;providing a resin in contact with at least one of the first layers orthe second layers; mating the first layer extending structure with thesecond layer extending structure to thereby form a transition zone, themating including interlacing the at least two first layers and the atleast two second layers into partially overlapping relation such that atleast one of the first layers is positioned adjacent to and between twosecond layers and extends in its plane beyond the two adjacent secondlayers, and at least one of the second layers is positioned adjacent toand between two first layers and extends in its plane beyond the twoadjacent first layers; and curing the transition zone.
 19. The method ofclaim 18, further comprising providing a means for at least partiallyimmobilizing the fibres.
 20. The method of claim 18, further comprisingpre-consolidating the transition zone.
 21. The method of claim 18,wherein the fibres for at least one of the first layers and the secondlayers are selected from one or more of the group consisting ofprepregs, semi-pregs, woven or non-woven fabrics, mats, pre-forms,pre-consolidated pre-forms, individual or groups of fibres, tows,tow-pregs and a combination thereof.
 22. The method of claim 18, whereinat least a part of the resin is introduced by at least one of resininfusion, resin transfer moulding, and vacuum assisted resin transfermoulding.
 23. The method of claim 18, wherein the curing involvesproviding of a pressure and/or a vacuum on the transition zone.
 24. Themethod of claim 23, wherein the pressure and/or vacuum on the transitionzone commences before the curing of the transition zone.
 25. The methodof claim 18, wherein the first composite member and the second compositemember are joined together, substantially end-to-end, via the transitionzone, in a direction of respective longitudinal planes of the firstlayer and the second layer.
 26. The method of claim 18, wherein thefirst resin, the second resin, and the resin in contact with at leastone of the first or second layers are composed of a thermosetting resin,and wherein the first material has at least one physical property suchas a coefficient of thermal expansion or Young's modulus that isdifferent than the same physical property of the second material.